Thin film magnetoresistive sensors or heads have been used in magnetic storage devices (e.g., disk drives) for several years. Such a sensor includes a layer of magnetoresistive material which is conventionally referred to as the free layer. The electrical resistivity of the free layer changes in response to an external magnetic field. Thus, magnetically recorded information is detected by sensing electrical resistivity changes in the free layer.
The free layer is typically a ferromagnetic material having a low coercivity, such as a NiFe, CoFe or NiCoFe alloy, so that its magnetization (also referred to as magnetic moment) can change easily in response to changes in the external magnetic field being sensed. In addition, it is highly desirable that the free layer be in a single magnetic domain state. If multiple magnetic domains, or vortex domain states, are present within the free layer, sensor performance will be degraded due to Barkhausen jumps and other undesirable magnetic domain motion and reorientation phenomena induced by the external magnetic fields to be sensed.
In order to ensure the free layer remains in a single magnetic domain state, a magnetic bias for the free layer is typically provided by bias structures adjacent to the free layer. These bias structures are usually made of hard (i.e., high coercivity and high magnetic moment) ferromagnetic materials, such as CoPt, and CoCrX alloys. Here X can be Pt, Ta, Ni or other elements.
FIG. 1 shows a typical bias configuration for a magnetic sensor free layer. A free layer 10 is biased by bias layers 12 and 14. Magnetizations 18 and 20 of bias layers 12 and 14 are typically set by application of a biasing magnetic field to the entire structure including layers 10, 12, and 14 at a relatively late stage of assembly. The biasing magnetic field has a field strength exceeding the coercivity of bias layers 12 and 14, so that when the biasing magnetic field is removed, remanent magnetizations 18 and 20 in bias layers 12 and 14 remain. Thus bias layers 12 and 14 act as permanent magnets for biasing free layer 10.
Magnetizations 18 and 20 of bias layers 12 and 14 induce a magnetization 16 in free layer 10. Magnetization 16 can be induced in free layer 10 by the process of magnetic exchange coupling, if free layer 10 is in direct contact with bias layers 12 and 14 (as shown on FIG. 1). Alternatively, magnetization 16 can be induced in free layer 10 by the process of magnetostatic coupling, if free layer 10 is not in direct contact with bias layers 12 and 14. Magnetization 16 should be large enough to ensure that free layer 10 remains in a single-domain state. However, magnetic sensor sensitivity decreases as the magnetic bias increases, so magnetization 16 is typically chosen to provide a suitable margin over the minimum required to force free layer 10 into a single-domain state.
In operation, an electrical current (not shown on FIG. 1) is typically passed through free layer 10 in the Y direction on FIG. 1, so that changes in resistivity of free layer 10 can be monitored. Therefore, magnetization 16 is frequently referred to as a longitudinal magnetization because it is in the same direction as this electric current.
Since magnetization is a vector quantity, having both a magnitude and a direction, magnetizations 16, 18, and 20 are to be understood as Y-components of the magnetizations in the corresponding regions (i.e., 10, 12 and 14 respectively). In practice, it is typically not possible to completely control magnetization direction, and the resulting variability tends to have a significant effect on performance.
FIG. 1 shows a view of layers 10, 12, and 14 as seen looking up from a magnetic recording disk (i.e., the disk is in the X-Y plane of FIG. 1). Furthermore, a track on the disk moves in the X direction on FIG. 1 as the disk rotates. Since the X extent of free layer 10 largely determines the density of information that can be read from the track, reduction of the X extent of free layer 10 is a primary goal as disk drive technology evolves. The other dimensions of free layer 10, and the dimensions of bias layers 12 and 14 also tend to decrease as disk drive technology evolves. For example, typical present day (X, Y, Z) dimensions for free layer 10 are about (3 nm, 100 nm, 100 nm), and typical present day (X, Y, Z) dimensions for bias regions 12 and 14 are about (3–15 nm, 30 nm, 200 nm).
The ever-decreasing dimensions of free layer 10 and bias layers 12 and 14 have led to the appreciation of new problems in small bias layers which are either absent or not as apparent in larger structures. One such problem is statistical variability in performance due to crystal grain structure and orientation within bias layers 12 and 14. This leads to variations of the magnetization direction of the individual grains comprising the bias layers 12 and 14.
FIG. 2 shows crystal grains 13a, 13b, 13c, and 13d within bias layer 12 of FIG. 1, and also shows crystal grains 15a, 15b, 15c, and 15d within bias layer 14 of FIG. 1. Crystal grains 13a–d have corresponding magnetizations (Y-components) 18a–d, and crystal grains 15a–d have corresponding magnetizations (Y-components) 20a–d. Magnetizations 18a–d and 20a–d typically vary from grain to grain, as indicated by the variable number of arrows within each crystal grain on FIG. 2. More precisely, the variable number of arrows within each crystal grain of FIG. 2 schematically indicate the variable contribution of each grain to longitudinal magnetization 16 of free layer 10. The contributions of the grains to magnetization 16 can vary due to a variable magnitude and/or direction of magnetization within the grains.
The main reason for variability of magnetizations 18a–d and 20a–d is that materials typically used for bias regions 12 and 14 are magnetically anisotropic and are typically deposited as polycrystalline films having grains with random orientations. For example, CoPt is easy to magnetize along the crystal c axis, and is more difficult to magnetize in other directions. The larger the angle between the magnetization direction and the crystal c axis, the more difficult CoPt is to magnetize, since all basal plane directions (i.e., directions perpendicular to the c axis) are hard magnetization directions.
On FIG. 1, the growth direction is the +X direction, and materials are typically deposited as layers in the Y-Z plane. Bias layers 12 and 14 are typically formed by deposition techniques, such as sputter deposition or ion beam deposition which do not inherently provide perfect control over crystal grain orientation. Therefore, unless further steps are taken, the grain orientation within bias layers 12 and 14 is entirely random. Methods for reducing the randomness of gain orientation are known, such as deposition of layers 12 and 14 on top of a suitable seed layer (such as Cr or a Cr containing alloy). However, introduction of a seed layer typically does not completely remove the randomness of grain orientation, at least in the Y-Z plane (i.e., the growth plane). For example, in CoPt grown on top of Cr, the c axis of the CoPt grains is constrained to lie within the growth plane by the Cr seed layer, but is random within this plane. This is achieved by lattice matching the atomic spacing of the seed layer to the atomic spacing of a plane including the c-axis of the hard bias layer material.
Thus, with or without the use of a seed layer, when magnetizations 18a–d and 20a–d are set by the biasing magnetic field in this example, remanent magnetizations 18a–d and 20a–d vary depending on the angle between the crystal c axis of grains 13a–d and 15a–d and the direction of the biasing magnetic field (i.e., Y on FIGS. 1 and 2).
The variability of magnetizations 18a–d and 20a–d of FIG. 2 undesirably leads to variability in magnetization 16 in free layer 10. As the number of grains contributing to magnetization 16 decreases, the relative standard deviation (i.e., the standard deviation divided by the mean) of magnetization 16 increases, since an average is effectively being taken over the number of grains which contribute to magnetization 16. Typical grain sizes are no smaller than about 7–10 nm in lateral (i.e., Y-Z plane) extent, since grains which are smaller are known to have undesirably reduced stability. Thus the number of grains in bias layers 12 and 14 decreases as the physical size of bias layers 12 and 14 decreases, thereby undesirably increasing the variability of magnetization 16 in free layer 10.
Variability of magnetization 16 has undesirable consequences in manufacturing. To illustrate, let MO be the minimum magnetization 16 required to force free layer 10 into a single domain state, and let M be the nominal design magnetization 16. A population of manufactured devices will exhibit a distribution of values for magnetization 16, centered on the nominal value M. If M is chosen to be just above M0, then a significant fraction of the population will fail due to insufficient magnetization 16. If M is chosen such that relatively few members of the population fail due to insufficient magnetization 16, then many members of the population will have unnecessarily reduced sensitivity due to magnetization 16 being substantially higher than is required.
FIG. 3 shows another known configuration, as taught in U.S. Pat. No. 5,434,826, for biasing free layer 10 of a magnetic sensor. In the configuration of FIG. 3, bias layers 12a and 12b are separated by an interposing layer 24, and bias layers 14a and 14b are also separated by an interposing layer 24. Magnetizations 18a–b and 20a–b are set within bias layers 12a–b and 14a–b respectively, and cooperatively provide magnetization 16 to free layer 10.